1. Field of the Invention
This invention relates to optical waveguide devices, elements for making the devices and methods of making the devices and elements. The devices are for interconnecting optical fibers, optical components and modules and for use in integrated optical systems.
2. Description of the Prior Art
In optical communication systems, messages are transmitted by carrier waves of optical frequencies that are generated by sources such as lasers or light-emitting diodes. There is much current interest in such optical communication systems because they offer several advantages over conventional communication systems, such as having a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages. One such means for conducting or guiding waves of optical frequencies from one point to another is called an optical waveguide. The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus, producing a guiding effect. The most frequently used material for such a waveguide device is glass, which is formed into a fiber of specified dimensions.
As the development of optical circuits proceeded, it became necessary to have devices which could couple, divide, switch and modulate the optical waves from one waveguide fiber to another.
Some optical fibers are interconnected by other optical fibers cut to length. These devices have only two terminals--one at each end. Photohardened films containing a waveguide have been proposed for this use, such as in U.S. Pat. No. 3,809,732. However, the device disclosed therein isn't easily coupled to and aligned with an optical fiber. Further, due to the uneven surface of its film, one cannot easily protect its exposed surface from the environment.
Another method used to form an optical coupling device involves the application of standard photolithographic processes and diffusion. By this prior art process, standard lithographic processes are used to define a pattern in a photoresist layer deposited on a chosen substrate. Then, an etchant is applied to etch the photoresist-defined pattern into the substrate. Next, a metal is deposited in the etched region by vacuum deposition. The photoresist pattern is then lifted off with an appropriate solvent, carrying with it unwanted metal deposits. The structure is then heated to diffuse the metal deposited in the etched region into the substrate, to form a waveguiding layer therein. See, for instance, U.S. Pat. No. 4,609,252. In addition to the fact that many steps are involved in such a process, there is also a limitation on the thickness of the metal which may be deposited. First, since vacuum deposition is a relatively slow process, there is the limitation of the excessive amount of time required to deposit a thick layer of metal. Secondly, as more and more metal is deposited, new centers for deposition are created, resulting in an uneven deposit.
To form branches, two or more fibers have been bonded to a common optical port using an adhesive having an index of refraction closely matched to that of the fibers. The fibers are very small in diameter and must be handled with extreme care, bundled together for strength, and attached to a support at intervals. Fabrication of the equivalent of a printed circuit board comprised of these discrete fibers and optical devices is labor-intensive, expensive, slow and tedious, and not readily adapted to automated fabrication techniques. Another method used to form such a coupler is to fuse or melt fibers together so that light from one fiber can pass to the connected fibers. However, in such a fusion process it is difficult to control the extent of fusion and the exact geometry and reproducability of the final structure.
A device of particular interest is the "Y-coupler", which is a "y"-shaped device that couples signals together or divides them apart. "Y"-shaped devices have been made in a wet process by exposing a liquid photoactive layer to ultraviolet radiation through a mask. Then a solvent is used to remove the unpolymerized portions of the layer. See, for instance, U.S. Pat. No. 4,609,252. The waveguide of this device, like those mentioned above, isn't protected from the environment or readily coupled to an optical fiber. Further, being a wet process, it has the tendency of being messy and the problem of disposing of the spent solvent.
Another "Y"-shaped coupler device is disclosed in U.S. Pat. No. 4,666,236. It further discloses a device with one input branch and three output branches. These devices are also made by a wet process exposing a liquid photopolymer film to light to create a waveguide. The unexposed liquid film is dried and becomes part of the device. The film is further coated with a layer, such as an acrylic resin, to prevent deposition of dust and staining. Again, this process is wet and, thus, inherently messy.
A further disadvantage of prior art couplers is that power and light intensity is lost when light passes through them, particularly through devices with abrupt-bends with large branching angles due to scattering of light at the intersection of the branches. Tsutsumi et al., (J. Lightwave Technol., LT-6, pages 590-600, 1988) have shown that the transmitted power of a Y-coupler will deteriorate rapidly as the branching angle increases above one degree.
U.S. Pat. No. 3,809,686 shows waveguides created in a single photopolymer film by focusing a beam of light within the film and moving the film. It shows multiple waveguides in a single film. In one embodiment, the waveguides exhibit evanescent coupling of light between the waveguides. It further teaches the creation and use of holographic diffraction gratings as light couplers. However, it is difficult to focus light within a film to form a homogenous waveguide with clear and distinct boundaries.
An object of this invention is to provide an improved method of forming conductors of light for interconnecting optical components and modules. These light conductors may be formed with a plurality of branches and terminals.
A further object of this invention is to provide improved optical waveguide devices, elements for making the devices and methods of making the devices and elements.
Another object of this invention is to provide an optical waveguide device including at least one high diffraction efficiency volume phase grating.
Another object of this invention is to provide a low power and light intensity loss splitter and reverse splitter.